The present invention relates to a ball-and-roller bearing, more particularly, to an improvement in the life of ball-and-roller bearings that are to be used in transmissions, engines, etc. on automobiles, agricultural machines, construction machines, as well as iron- and steel-making machines.
One of the factors that have heretofore been known to cause a shortened life of ball-and-roller bearings is the entrance of foreign materials into bearing lubricants. It is known that metal cuttings, shavings, burrs, worn particles and other foreign materials are present in lubricants for bearings. When ball-and-roller bearings are used in an environment that involves such foreign materials, the latter will damage their races and/or rolling elements, thereby shortening their service life. The problem with this situation is that in the presence of foreign materials, the service life of ball-and-roller bearings is shortened to one-tenth of the case where they are absent from lubricants.
Further, as in the case of pitting which occurs in the o automotive speed change gear wheels, specks of pits develop on the raceway surfaces or rolling contact surfaces of ball-and-roller bearings due to the fatigue caused by rolling, thereby shortening the life of those bearings. Under the circumstances, Examined Japanese Patent publication No. 24499/1987 and Unexamined Published Japanese Patent Application No. 34766/1990 have proposed that a heat treatment such as carburization be conducted on low- to medium-carbon, low-alloy steels to precipitate spheroidal carbides on their surface, thereby enhancing the hardness of their surface so that they will have better resistance to pitting.
This prior art method which relies upon enhancing the surface hardness of the races and rolling elements of a ball-and-roller bearing is effective in reducing the formation of indentations due to foreign materials. On the other hand, the races and rolling elements will have lower toughness and cracking tends to develop at sites of damage that is caused by the foreign materials present in the lubricant; this has been the start point of early flaking, which therefore limits the extent by which the service life of ball-and-roller bearings can be prolonged.
Ball-and-roller bearings are also subjected to cyclic shear stresses under high areal pressure and, hence, bearing materials are required to have sufficient strength to withstand the applied shear stress. To this end, high-chromium carbon bearing steels have heretofore been used. However, as the conditions under which bearings are used have recently become increasingly hostile, particularly in the case of lubrication in the presence of foreign matter, difficulty is involved in extending the life of bearings by merely increasing their mechanical strength, chiefly hardness.
Under the circumstances, the following was previously proposed in Unexamined Published Japanese Application No. 55423/1989: if the contents of carbon, retained austenite and carbonitrides in the rolling surface layer of a ball-and-roller bearing are specified to appropriate levels, the stress concentration that will develop at edges of indentation due to a foreign material present in the lubricant for the bearing can be reduced and the occurrence of cracking can also be suppressed to extend the life of the bearing.
According to that proposal, the life of the ball-and-roller bearing under lubrication in the presence of a foreign material can be improved by insuring that retained austenire will be present in an appropriate amount. On the other hand, the proposal involves the disadvantage that the retained austenite lowers the surface hardness of the bearing, thereby reducing its fatigue resistance. In other words, the proposal had room for improvement to establish the appropriate relationship between the retained austenite level (.gamma..sub.R in vol %) and the surface hardness (Hv).
In addition, the proposal did not take into account the effects on the bearing's life of the particle size of carbides and carbonitrides; in particular, large-size carbides, if they are subjected to cyclic stress, will be the start point of fatigue, leading to the occurrence of cracking and flaking but this possibility was not at all considered by the proposal.
To solve this problem, an inventor of the present invention previously filed U.S. Pat. No. 5,137,375, establishing an optimal relationship between the content of retained austenite in the rolling surface layer of a ball-and-roller bearing and its surface hardness. Further, the average particle sizes of carbides and carbonitrides present in the rolling surface layer were adjusted to optimal values so as to compensate for the decrease in surface hardness due to the presence of retained austenite, thereby providing a long-lived ball-and-roller bearing.
The related art described in the U.S. Pat. No. 5,137,375 has had the problem that if the carbon content of a bearing material exceeds a certain level, macrocarbides tend to form and that, therefore, carburization at high carbon concentration can result in the formation of macrocarbides on the surface, leading to a shortened life of the bearing.
A further problem with the related art is that if the bearing material is subjected to a carburization treatment only, quenching must be done at elevated temperature in order to yield an appropriate amount of austenire. However, if quenching is performed at elevated temperature, the content of fine carbides will decrease to such an extent that the precipitation hardening effect of the fine carbides cannot be fully attained.